Hi
I think you are using Shopping Cart feature, is that correct ? If so, the extension won't work because when users purchase multiple documents within the same checkout, the extension doesn't know what Paypal account should receive payment, so it will default to site owner Paypal account. In this case, please go to Documents Seller -> Configuration, find the config option Activate Cart Feature, set it to No and it will work as expected .
Regards,
Tuan